1. Field of the Invention
This invention relates to an apparatus and a method for optically measuring the characteristics of a thin film particularly spread on a liquid surface, as well as to equipment provided with said apparatus, for forming a monomolecular built-up film. More particularly, the present invention relates to an apparatus and a method for measuring the light absorption characteristic of a thin film which provides a basic data for analysis of the characteristics of said thin film. The present invention is utilized in, for example, analysis of the characteristic of a monomolecular film spread on a liquid surface to be formed into a monomolecular built-up film.
2. Description of the Prior Art
Conventional apparatuses have been developed for measuring the light absorption characteristic of a sample, and for measuring said characteristic form the transmissivity or reflectivity of the sample. However, when a light is applied to a sample, there appears a scattered light in addition to a transmitted light and a reflected light. In order to conduct an evaluation of the light absorption characteristic of a sample with high precision, it is important to directly measure a light component absorbed by the sample.
Apparatuses for directly measuring the absorbed light component include photoacoustic spectroscopy (PAS) apparatus and a photothermal radiometry (PTR) apparatus utilizing a phenomenon that, when a light is intermittently applied to a sample, the light energy absorbed by the sample is intermittently converted to heat by a radiationless relaxation process.
The PAS apparatus is classified into a microphone type and a piezoelectric element type. In the microphone type a sample needs to be placed in a closed chamber, and in the piezoelectric element type, the arrangement of a detector and a sample is important. Any of these types are not suitable for the measurement of a thin film spread on a liquid surface. In the PTR apparatus which uses an infrared detector, the measurement is easily affected by the atmospheric conditions such as humidify or the like.
Another apparatus for directly measuring a light component that is absorbed is a photothermal deflection spectrosocpy (PDS) apparatus. This apparatus utilizes the phenomenon that the sample which has absorbed a light generates heat, which causes a temperature distribution within the sample and in its vicinity, and that this temperature distribution changes the refractive indexes of the sample and its vicinity and therefore the light coming into the sample is deflected.
In other words, in PDS apparatus, two kinds of light are projected onto the measurement site; one kind is exciting light which generates heat in the sample upon light absorption to cause the temperature distribution to change the refractive index and the other is probe light for detecting deflection of the exciting light caused by the above- mentioned change of the refractive index, and therefore the light absorption characteristic of the sample is measured from the length of the exciting light and the deflection of the probe light. The PDS apparatus is suitable for field measurement or remote measurement because a sample and a detection system can be arranged independently. The apparatus of the present invention is the same as the PDS apparatus in basic principle.
The PDS apparatus are classified into two types, a transverse type and a collinear type according to the arrangement of an exciting light and a probe light. Both types measure the amount of delfection of probe light corresponding to the amount of exciting light absorbed by a sample. The detector is often a position sensitive detector (PSD).
FIG. 10A illustrates an example of the PDS apparatus of the collinear type. An exciting light 11 emitted by an exciting light source 10 is converted into intermittent exciting light by a chopper 12 and is converged by a lens 34 and is projected to a sample 4'. Probe light 6 emitted from a probe light source 5 is allowed to pass the very site of the sample 4' to which the exciting light 11 is projected by means of a lens 35 and a light path regulator 17 such as a mirror or the like and then reaches a detector 7. For this probe light 6, the amount of deflection as shown in a dotted line is measured. FIG. 10B is an example of the PDS apparatus of transverse type. The only difference between this transverse type and the collinear type is that the probe light 6 is applied parallel to the surface of the sample 4'.
The theoretical treatment of the phenomena in the PDS apparatus can be made by solving the equation of heat conduction taking place in a sample. The amount of deflection as measured in terms of deflection angle is proportional to the intensity of exciting light, the temperature coefficient (.differential.n/.differential.T) of the refractive index, the temperature gradient (.differential.T/.differential.K) at the portion through which a probe light passes, etc. Terms proportional to the light absorption coefficient of the sample are contained in (.differential.T/.differential.K). (.differential.n/.differential.T) can take a positive or negative value depending upon the sample, which implies that the deflection angle, can be positive or negative.
The PDS apparatus, when applied to a measurement for a thin film spread on a liquid surface, has the following disadvantages because the film as a sample is extremely thin. "A thin film spread on a liquid surface" in this specification means a thin film extended over a liquid surface without sinking or floating, such as a monomolecular film.
In a thin film spread on a liquid surface, the path through which exciting light travels is short and consequently the exciting light is liable to be affected by external environments before the exciting reaches the liquid surface, such as dusts and fluctuation in the air, and so on. Moreover, the S/N ratio is reduced also by reflected light or transmitted light after the exciting light has reached the thin film. Thus, measurement with good precision and good sensitivity is hardly obtainable. In some cases, measurement is conducted in such a way that there is used, as a gas phase present over a liquid surface, a special gas which interacts with a thin film on the liquid surface. In such a measurement, it is necessary to make a portion of the gas through which the exciting light passes as short as possible; however, realization of such a measurement is difficult.